1. The Field of the Invention
The present invention relates generally to fluid bed granulators, coaters and dryers. In particular, the invention relates to a filter apparatus including a movable filter bundle that can be selectively raised and lowered to facilitate maintenance of one or more filter units on the movable filter bundle.
2. The Relevant Technology
Granulation is a crucial stage in many industries, such as mineral processing, agricultural products, detergents, pharmaceuticals, foodstuffs, and specialty chemicals. It is a size enlargement process where fine powder feed particles are bound together to form agglomerates or granules, a process that, for example, enables the formation of tablets.
Wet granulation, one type of granulation, is a process where the particles are bound together by a fluid (also called a binder) which forms liquid bridges between the particles to hold them together. The binding fluid is usually a solvent, like water or ethanol, or a solution of a polymeric binder in a solvent.
A fluid bed granulator is a tall cylindrical or rectangular vessel containing a bed of particles. Air is forced through a distributor at the base which fluidizes and agitates the powder. A binding fluid is added by spraying from above, below, or within the powder bed. These binder drops collide with the powder particles and form liquid bridges, which hold them together by capillary suction. By heating the fluidizing air, the product can be granulated and dried simultaneously, which is particularly useful in the specialty chemical and pharmaceutical industries.
Fluid bed granulators, coaters, and dryers create a large amount of dust that fills the process air used therein. In order to prevent the release of this dust into the environment these devices require the use of filter systems.
One such filter system is a bag filter system as illustrated in U.S. Pat. No. 5,446,974 to Gubler, herein incorporated by reference. The Gubler patent discloses a filter assembly having a fluid bed chamber wherein the process air enters from one end, is filtered by one or more bag filters, and exits the opposite end. The bag filters are rigidly attached to a mounting plate within the filter plenum during operation and are unattached and removed from the filter assembly for servicing.
The individual bag filters disclosed in the Gubler patent comprise flexible filter bags that are fitted over and clamped onto underlying rigid frames. During operation, the filter bags form around and through side bars of the rigid frame in an accordion shape. Periodic cleaning of the filter bags is achieved by applying a periodic pulse of reverse air to the bag filter, forcing the filter bag outward from the rigid frame and dislodging particulate matter that has accumulated on the filter bag. Although this is remarkably effective, the filter bag must be eventually removed and cleaned or replaced.
Of course, a variety of other filter assemblies, filters, and bag filters exist that can also be effectively used to filter process air. With each of these filter systems the degree of difficulty in cleaning or replacing a filter in a fluid bed system can increase costs and limit the operational time of the filter system and its corresponding fluid bed system.
U.S. Pat. No. 6,733,574, to Gubler, incorporated herein by reference in its entirety, teaches the use of a system for transporting a filter mount between a raised operative position and a lowered maintenance position that uses an index rod and associated structure to properly line up the filter mount when restored to the operative position after maintenance. This has been an effective approach for allowing simpler and faster maintenance than was found in prior art systems. However, it suffers from the tendency for the filter mount to spin and bang around as it is moved between the operative and maintenance positions.